Fungi with aseptate hyphae and no dikaryophase

FUNGI WITH ASEPTATE HYPHAE AND NO DIKARYOPHASE 

Contents 

John Brown 

4.7 Ptasmodiaphoromgcota......... ..................49 

4.2 Oomgcota......... .......51 

Saprotegniales ....,... ........... 52 

Sclerosporales..,......... ........ 52 

Pgthinles .............52 

Peronosporales........ ........... il 

4.3 C@tridiamgcota .....55 

4.4 Zggomgcota......... ..................56 

Mucorales ...........56 

Endogonates andGlamaLes .................58 

4.5 nrther reading ...... 59 

The fungi discussed in this chapter constitute a heterogeneous group spread over 

three kingdoms. The Zygomycota and Chytridiomycota are usually considered to 

be true fungi and placed in the kingdom Eumycota. The Oomycota (kingdom 

Chrornista) and Plasmodiophoromycota (kingdom Protoctista) are not true fungi 

and are sometimes referred to as pseudofungi. However, despite the lack of 

phylogenetic relationships between the four phyla, they have characteristics in 

common and all are studied by mycologists and plant pathologists. The features 

common to all four phyla are the production of aseptate hyphae (when hyphae 

are formed) and the absence of a dikaryophase in their life cycle. Three of the four 

phyla can form motile spores (zoospores) and are often referred to as zoosporic 

fungi. Tl:e Zygornycota do not form motile zoospores. 

4.1 Plasmodiophoromycota 

The Plasmodiophoromycota are placed in the kingdom Protoctista. Members of 

this phylum are obligate endoparasites of vascular plants, algae and fungi. They 

frequently induce hypertrophy (giant cell production) and hyperplasia 

(uncontrolled cell multiplication) resulting in the formation of galls in their hosts. 

TWo species that affect plants are Plasmodiaphorabrassicae which causes club 

root of brassicas and Spongospora subterranea the cause of powdery scab of 

potatoes. PoLgmgxa gramints, which lives necrotrophically (. necrotroph is a 

parasite that derives its energy from dead host cells) on the roots of grasses is a 

vector of at least seven cereal and grass viruses. 

Plasmodiophora brassicae produces resting spores or cysts in the giant cells of 

root galls. These resting spores are released into the soil when the root system of 

infected plants rots away and can survive in soil for 20 years or more. Resting 

spores germinate to form a single primary zoospore possessing two unequal, 

anterior, whiplash flagella of simpler structure than those of Oomycota and 

Chytridiomycota. These zoospores, which become intermittently amoeboid, swim

50 JohnBrotun 

in films of water in the soil's pore spaces and come to rest and encyst (stop 

swimming, withdraw their flagella and lay down a thick wall) when ttrey contact 

the root hair of a host plant. The cyst produces a specialised tubular structure 

that pierces ttre host's cell wall so the protoplasm of the fungus can pass into the 

host cell. This protoplasm develops into a plasmodium (a wall-less mass of 

protoplasm containing numerous nuclei). After a few days, the plasmodium 

divides into uninucleate units, each of which becomes surrounded by a separate 

membrane. The nucleus within each unit divides mitotically and each unit is 

converted into a zoosporangium containing +16 secondary zoospores. These 

zoospores are released and either remain in the root or move out into the soil. 

The function of these zoospores is uncertain but they probably. function as 

gametes and fuse to produce new cells which then reinfect the host cells to 

produce new diploid plasmodia. At maturity, plasmodia divide into uninucleate 

portions which develop into haploid resting spores after undergoing meiosis. A 

probable life cycle for P. brassicae is shown in Figure 4.1. 

Resting spores 

in giant cells 

@ 

*".,'ng sPore in soil 

Single prirnary zoospore 

G 

Amoeboid stage 

Figure 4.1 

t-.p 

Secondary 

Deconqary 

zoospores 

acting tng a / re -z 

2Q 

fl2MATAC 

,/ 

Life cycle of Plasmodiophora brassicae, the cause of club-root of Brassica 

spp. 

Ttre root galls caused by P. brossicae restrict movement of water and nutrients 

from the soil to the above ground parts of plants. This leads to wilting during hot 

periods of the day as well as yellowing and shedding of lower leaves. The disease 

is most destructive in brassica crops but has also been observed in other plants 

including clover, ornamentals and strawberryr. Spread of the disease to new areas 

depends on movement of infected plants, particularly seedlings, and movement of 

resting spores in soil (e.g. on farm machinery).

4. Furgi with aseptate hgphae and no dikaryophase 

5l 

4.2 Oomycota 

The Oomycota differ from other groups of fungi in several ways. 

. Flagella, when present on zoospoores, are of two types. Whiplash flagella, have 

smooth continuous surfaces and are directed baclcrvards while the zoospores 

are swimming. Tinsel flagella, have two rows of hair-like processes called 

mastigonemes on their surfaces and are directed forward during swimming 

(Fi9.4.2). 

r Two morphologically distinct Wpes of zoospores occur in some species (e.g. 

Saprolegnia spp.) Primary zoospores released from the undifferentiated 

sporangia are pear-shaped and have anterior flagella. Secondar5r zoospores, 

which arise from the encysted primary zoospores, are kidney shaped and have 

lateral flagella. Species that form two types of zoospores during asexual 

reproduction are said to be diplanetic (Fig. 4.2). 

flagella 

./l 

iplash f lagella 

(E IB) 

Flgure 4.2. 

Diagrammatic representation of the two types of zoospores produced by 

some members of the Oomycota. (A) Primary zoospore. (B) Secondary 

zoospore. 

. Sexual reproduction is oogamous with one or more non-motile oospheres 

produced inside an oogonium. Fertilisation occurs when antheridia become 

attached to the outer wall of the oogonium and fertilisation tubes penetrate the 

wall to fertilise the oospheres. The fertilised oospheres lay down a thick wall 

and are converted into oospores. Oospores of some species can survive for 

several years in a dormant state in soil or water. 

. The hyphae are aseptate and, unlike other groups of fungi, the cell walls are 

composed primarily of cellulose and -6-glucan and rarely contain chitin. 

. The assimilative phase is diploid whereas in all other groups of fungi it is 

either haploid or haploid dikaryotic. 

The oogamous nature of sexual reproduction, the presence of cellulose in the 

cell walls and the presence of tinsel flagella indicate that the Oomycota are not 

true fungi and are probably related to algae that also have tinsel flagella. They are 

therefore placed in the kingdom Chromista. 

Economically, the most important members of the Oomycota are plant 

parasites especially root rotting fungi, the downy mildews and the white blister 

fungi. Some species also attack fish (e.g. SaproLegnia parasitba) and crayfish (e.g. 

Aphanomyces spp.) particularly when they are farmed. Many species are 

saprophytes in freshwater, marine and terrestrial environments. 

TWo plant diseases of socio-economic importance caused by fungi belonging to 

the Oomycota, were introduced into Europe in the 184Os from the Americas. 

Phgtophttnra i4festans, the cause of late blight of potato, affected the patterns of 

human migration in the I8OOs, particularly emigration from Ireland, and 

Ptasmapara uiticola, the cause of downy mildew of grape vines, led to the chance

52 JohnBrown 

discovery of Bordeaux mixture (copper sulphate + hydrated lime + water) which 

was the first fungicide to be widely used by farmers. 

Hawksworth et al (1995) recognise nine orders within the Oomycota. However 

only four will be considered here-Saprolegniales (e.€. Aphanomgcesl 

Sclerosporales (e.g. downy mildews of grasses), Srthiales (e.g. Pgthiumand 

Pttgtophttwra) and the Peronosporales (the downy mildews of dicots). 

Saprolegniales 

The only genus in the order Saprolegniales that is an important plant pathogen is 

Aphanomgces which causes root rot of many annual plants (e.9. A. euteictrcs on 

pea and other legumes and A. cochlioides on sugar beet). 

Aphanomgces euteiches produces asexual zoospores in an undifferentiated 

sporangium. The primary zoospores that are released aggregate at the mouth of 

the sporangium where they encyst. The secondary zoospores that subsequently 

emerge from each encysted primary zoospore swim for a time and then encyst. 

The secondary cyst germinates to form an aseptate, hyaline mycelium which 

infects the plant. 

Oogonia and antheridia form in abundance on the tissues of diseased plants. 

After fertilisation a single oospore develops within each oogonium. Oospores are 

18-25 pm in diameter and can survive in a dormant state in soil for many years. 

They germinate by producing one or more germ tubes or by producing a long, 

slender hypha which develops into a spor€rngium containing about 15 zoospores. 

Both tJle germ tubes and zoospores can infect plant roots. 

Sclerosporales 

Members of this order are parasites of grasses and cause downy mildew and root 

rot diseases. Their narrow hyphae (less than 5 pm in diameter) invade their hosts 

and form thick-walled, warty oogonia. Two families are recognised- 

Sclerosporaceae and Vermcalvaceae. 

Members of the Sclerosporaceae are obligate parasites that form intercellular 

hyphae with peg-like haustoria in their hosts. They cause downy mildew 

diseases. Sporangiophores are more or less dichotomously branched. TWo genera, 

PeronoscLerospora" and Sclerospora belong to this family. 

Members of the Vermcalvaceae are parasites of grasses that can be cultured 

on artificial media. Their mycelium does not form haustoria and sporangiophores 

are poorly differentiated. Three genera are recognised Pachymetra, Sclerophthora 

arrd Verntcaluus. 

In northern Queensland, Pachgmetra chaunorhizarots the roots of sugarcane, 

particularly the root tips. In susceptible plants, infection results in a decrease in 

plant vigour, leaf yellowing and poor stooling (regeneration from the base of the 

plant). Affected plants are pulled from the ground during mechanical harvesting 

operations because their weakened root systems do not anchor them in the soil. 

Kikuyu (Pennisehtm clandestinum), an important pasture grass, suffers from a 

disease known as kikuyu yellows in northern New South Wales. The disease is 

caused by Verrucaluus JlauoJaciens which causes a root rot resulting in plants 

becoming yellow and unthrifly. 

Pythiales 

The order $rthiales contains numerous saprophytic species but is best known for 

the 'damping-off and root and foot rot pathogens, Pgthium xfi Phgtophttnra spp. 

Infection by Pgthirm spp. is commonly characterised by soft, watery host tissues

4. nnWitaith aseptate hgptwe andno dtkaryophase 

53 

containing coarse, intracellular hyphae (usually 6-10 pm in diameter) which 

usually lack haustoria. Although considerable variation occurs in the asexual 

and sexual reproductive processes of Pythium spp., the life cycle of Pgthium 

debargarturn, which lives in soil as a saprophyte on dead organic matter but can 

also infect seedlings and cause damping-off, is typical of the genus (Fig. 4.3). 

r,r'. tl-( 

,z K4.t - . / \ \t 

/ 

\ 

/ Zoospores r, 

f 

6D 

\ 

/\( 

L U @ 

Germinat:orOo"po.eY,, 

giumf\ 

V n 

/) lr 

lqz 

Figiure 4.3 Life cycle of Pgthiumdebaryanum- (From Parbery, 1980.) 

Pgthium produces a white mycelium that grows rapidly giving rise to terminal 

or intercalary sporangia that may be spherical or filamentous. Sporangia 

germinate by producing one to several germ tubes, or by producing a short hypha 

on the end of which a vesicle is formed (Fig. 4.3). Protoplasm passes into the 

vesicle and becomes differentiated into numerous zoospores (a single vesicle can 

contain over lOO zoospores). The zoospores released from the vesicle are 

secondary zoospores which come to rest and encyst shortly after their release. 

The encysted zoospores germinate by producing a germ tube which can infect 

susceptible hosts..Sexual reproduction is similar to that of AptwnomAces and is 

illustrated in Figure 4.3. 

Pgthium spp. infect seed, young seedlings and roots as well as the soft fleshy 

tissue of fruits and vegetables. The fungus secretes enzJrmes which dissolve the 

middle lamella and cell walls of host cells causing disruption and death of host 

tissue. Pgthtum diseases in greenhouses can be managed by partially sterilising 

soil by steam, dry heat or volatile chemicals. Host resistance to unspecialised 

pathogens such as Pgthium is usually not available. Treatment of seed and 

vegetative propagating material with fungicides has been used to reduce the 

incidence of pre- and post-emergent damping-off. Cultural practices (e.g. good 

soil drainage) and biological control agents have also been used to reduce disease 

incidence. 

Phgtophtttora spp. can be distinguished from Pgthium spp. in that no 

sporangial vesicles are formed or, if formed, the zoospores are mature when they 

enter the vesicle. Sporangia are often pyriform (pear-shaped) and develop on 

simple or branched sporangiophores. The well-known late blight pathogen of 

potato (Phgtophthora rgfestans) forms sporangia which function as wind 

dispersed conidiosporangia. Depending on microclimatic conditions, the 

sporangia germinate either by germ tubes or by producing zoospores. Sexual 

reproduction in Phytophttwra is similar to that of Pgthiumand Aptnrnmyces and 

results in the formation of oospores.

54 JohnBrousn 

Species of Phgtophthora cause a variety of diseases in a wide range of plant 

species. Most species cause seed rots as well as foliar and flower blights. Some 

Phgtophtttora spp. that occur in the Australasian region (Australia, New Zealand 

and neighbouring Pacific Island countries) and the diseases they cause include: 

P. cactorum 

P. cinnamomi 

P. citricola 

P. citrophttnra 

P. clandestina 

P. colocasiae 

P. cryptogea 

P. eryttvoseptica 

P. inJestans 

P. megasperma var. sojae 

P. nicotinnae 

P. palmiuora 

P. uignae 

collar rot of apple 

root rot in a wide range of plants including avocado, 

pineapple and many Australian native plants 

butt and fruit rot of citrus 

collar rot of citrus 

decline in subterranean clover 

leaf blight of taro 

foot rot of a wide r€rnge of plants including ornamentals 

pink tuber rot of potato 

late blight of potato and tomato-not a serious problem in 

Australia 

root rot and wilt of soybean 

collar rot in a wide variety of plants including ornamentals 

black pod and canker of cocoa 

stem rot of cowpea 

Peronosporales 

Many of the downy mildew fungi and all of the white blister fungi are placed in 

the order Peronosporales. A11 are obligate parasites of plants and form an 

intercellular network of hyphae that produce well-developed haustoria in infected 

tissue. The order Peronosporales consists of two families: the Peronosporaceae 

(many of the downy mildews) and the Albuginaceae (white blister or white rust 

fungr). 

Peronosporaceae (the downy mildews of dicots) 

The downy mildews cause foliage blights of dicots (except Peronospora destrtrctor 

which infects onions) and are characterised by the presence of a white or grey 

downy growth on the lower surface of leaf or stem lesions. This downy 

appearance is caused by the production of sporangiophores and sporangia on the 

host's surface. The sporangia are wind dispersed and germinate in water films on 

plant surfaces to release secondary zoospores. sporangia of the genus 

Peronospora do not release zoospores but germinate by producing a gerrn tube. 

Sexual reproduction in the downy mildews is similar to that described for 

Pgthiurn debaryartum (Fig. 4.3). 

Genera of downy mildews are identified by the type of branching of their 

sporangiophores which usually emerge from the host through stomata. 

Representative sporangiophores of five downy mildews are shown in Figure 4.4. 

Downy mildew fungi found in Australia include Bremia Lactucae (lettuce), 

Peronospora para"srtica (brassicas), P. tabacina (blue mould of tobacco), 

P. trdoLiorum (clovers and medics), P. uiciae (peas), P. destructor (onions) and 

Plasmopara uiticoLa (grapevines). Downy mildew of sunflower caused by 

PLasmoparahabtediiis a serious disease in most regions of the world but has not 

yet been observed in Australia. However, a strain of P. halstedii has been found 

on cape weed (Arctottteca caLendttla). 

Albuginaceae (the white blister fungi) 

The family Albuginaceae consists of a single genus, Albugo, which contains about 

3O species of obligate plant parasites. Sporangia are borne in chains in blister-

4. F-urEi usith aseptate hgphae artd no dikaryophase 

55 

like lesions which usually occur on the undersurface of leaves or on stems. 

Because the sporangia appear white and powdery on the plant's surface the fungi 

are often referred to as the white rust fungi. Oospores are produced in the 

infected host tissue and appear to have a survival function during intercrop 

periods. Two species common in Australia are A. candida on brassicas and 

A. tragopogonis on sunflower and a few other plants in tJle family Asteraceae. 

Plasmopara Sclerospora Basidioptwra 

Figlrre 4.4 

Sporangiophores and sporangia of five downy mildew fungi. (From Parbery, 

1980.) 

4.3 Chytridiomycota 

The phylum Chytridiomycota contains a few plant patlrogens including Otpidium 

brassicae, which infects the roots of a wide variety of plants but does not usually 

cause extensive damage. It is however a vector of about six plant viruses 

including tobacco necrosis virus and lettuce big vein virus. Another member of 

the phylum, Synchgtrium endobioticurn, causes black wart disease of potato and 

is present in most potato growing areas of the world. However, it has not been 

found in Australia and is restricted to one area of New Zealand. The disease is 

characterised by the formation of large, black galls on the base of stems and on 

tubers. The fungus can also be a vector of viruses such as potato mop top virus. 

Phgsoderma aLfaLJae causes crown wart of lucerne and other medics 

characterised by the formation of galls in the crown region of the plant. The 

disease occurs in some parts of Australia and New Zealand. 

Many Chytridiomycota are widespread in water and soil and play an important 

ecological role as decomposers of drganic material. Many can utilise organic 

substrates such as cellulose, hemicellulose, chitin and keratin. A few species are 

obligate anaerobes in the gut of herbivores (e.g. ruminants and marsupials) 

where they assist in the digestion of cellulose and other plant structural 

polysaccharides. Some genera such as CoeLomomAces are parasites of insects 

including mosquitoes and are attracting attention as potential biocontrol agents. 

Other species parasitise algae, fungi and small animals.

56 JohnBrown 

Members of the Chytridiomycota have aseptate, coenocytic hyphae (when 

hyphae are formed), chitinous-f-glucan cell walls, flat mitochondrial cristae and 

form zoospores, usually with a single, posterior whiplash flagellum. A few species 

(e.g. some anaerobic gut fungi) have more that one flagellum. 

The Chytridiomycota contains a single class, the Chytridiomycetes. In the past 

the Chytridiomycetes were classified on the basis of their morphologr and the 

substrate on which they grew. However, it is now known that morphological 

characters are extremely variable and of little use in classification. Nowadays the 

ultrastructure of the zoospore forms the basis of classification and this can only 

be seen using a transmission electron microscope. However, mycologists who 

specialise in this group can detect major differences between zoospores using a 

light microscope. 

4.4 Zygomycota 

The Zygomycota form well-developed aseptate mycelia and produce non-motile, 

asexual sporangiospores inside sporangia. Sexual reproduction, when it occurs, 

is by gametangial conjugation between two compatible gametangia which results 

in the formation of a thick-walled, often ornamented, zygospore. Zygospores along 

with chlamydospores serve as resting spores. The zygospore stage, which 

constitutes the teleomorph of the fungus, is not associated with a complex 

fruiting structure as occurs in the Ascomycota and Basidiomycota. 

The Zygomycota is divided into two classes, the Zygomycetes and the 

Trichomycetes. The Trichomycetes are associated with living arthropods and will 

not be considered here. The Zygomycetes are a small group of fungi consisting of 

less than 9OO described species. They are fast-growing primary colonisers of 

organic substrates, especially those containing simple carbon sources such as 

sugars and starch. A few species cause diseases of plants and animals. Seven 

orders are recognised within the Zygomycetes but only three will be mentioned 

here, the Mucorales, the Endogonales and the Glomales. 

Mucorales 

Most members of the Mucorales are saprophytes on plant debris. A few genera 

(e.g. Rhizopus) cause soft rots of fruits and vegetables. Rhizopus ohgosporus is 

used to produce tempeh-a fermented Asian food made from soybeans. Rhizoptts 

stoLonifer is known as common or black bread mould because it produces black 

sporangia when growing on bread. Some genera (e.g. Piptocephatis and 

Sgrrceptmlis) are obligate parasites of other Zygomycetes. 

Sexual reproduction in the Mucorales 

Sexual reproduction takes place when two compatible, specialised hyphae are 

attracted to one another and fuse. This attraction is controlled by chemicals 

secreted by the hyphae. The tips of the hyphae then swell to form progametangia 

{s. progametangium). A septum forms near the tip of each progametangium 

dividing it into two cells, a terminal gametangium and a suspensor cell. The 

wall between the fused gametangia dissolves and the protoplasts of the two cells 

mix. Nuclear fusion or karyogamy takes place immediately after plasmogamy 

(protoplasmic fusion) to form a diploid nucleus. No dikaryophase occurs. The cell 

that forms after nuclear fusion enlarges into a thick-walled zygospore. Meiosis 

occurs before the 4rgospore germinates so that the resulting colony is haploid. 

The haploid state dominates the life cycle of. Zygomycetes.

4. F\ngi uith aseptate hgphae and no dikaryophase 

57 

In some species the gametangia and suspensors are morphologically similar 

(e.9. Rhizopus and Mucor) while in others they are different (e.g. Absrdia and 

Zggorhgnchus). Some genera have appendages arising from one or both 

suspensors (e.g. Absidiaartd Phgcomgces) (Fig. 4.5). 

Zygospores, which act as resting spores, often exhibit a period of dormancy 

before they germinate. In some species (e.9. Rhizopus stoLonder) the 4rgospore 

germinates to form a single sporangium which is called a germ sporangium. In 

other species zygospores germinate to produce a haploid mycelium. 

Figure 4.5 

Some examples of morphological variation among teleomorphs of the 

Mucorales. (A) Rhrzopus stolonifer. (B) Zagortrynchus sp. (C) Phgcomgces sp. 

(From Parbery, I9BO.) 

Asexual reproduction in the Mucorales 

In their natural environment, most members of the Mucorales rarely produce 

zygospores. Reproduction is predominantly by asexual sporangiospores. Many 

species also form chlamydospores. 

The morphologr of sporangia and sporangiophores varies among members of 

the Mucorales and can be used to distinguish genera. Rhizopus stoLontfer 

produces two types of hyphae, non-reproductive assimilative hyphae which 

penetrate the substrate on which the fungus grows and stoloniferous hyphae 

which form rhizoids (root-like anchoring hyphae) at their point of attachment to 

the substrate. At these points of attachment (the nodes) groups of 

spor€rngiophores and sporangia are formed (Fig. 4.68). The genus Absidiadiffers 

in that the groups of sporangia are formed at an internodal position. Many genera 

{e.9. Mtrcor) do not produce rhizoids and stoloniferous hyphae. In Mucor the 

sporzrngiophores arise singly from the mycelium and may be branched producing 

several sporangia or unbranched forming a single, terminal spora.ngium. 

The genera Absidia, Mucor and Rttizoptm produce what is referred to as mucortype 

sporangia with a dome-like extension (the columella) of the sporangiophore 

extending into the sporangium (Fig. 4.6A). Genera such as Mortierella do not form 

a columella. Sporangia contain large numbers of non-motile sporangiospores 

which are released when the sporangial wall dissolves or is ruptured by outside 

forces. 

Some members of the Mucorales produce sporangioles which are small 

sporangia-like structures containing one or a few sporangiospores. Morphological 

differences in sporangia, sporangioles and the sporangiophores which bear them 

can be used to distinguish genera {Fig. 4.6). For example, Thamnidium spp. 

produce a terminal mucor-t5rpe sporangium and sporangioles on dichotomously 

branched sporulngiophores. choanephora. spp. and cunninghameLla spp. have 

branched sporangiophores which terminate in spherical heads on which 

sporangia containing a single spore are formed. In Sgncephalastrum the 

sporangiophores terminate with a swelling on which sporangioles containing a 

single row of spores are formed (Fig. 4.6).

58 JohnBrown 

Figure 4.6 

Sporangiophores and sporangia of Mucorales. (A) Mucor racemosus, (i) 

sporangium with columella and (ii) dehisced sporangium with two 

chlamydospores within the sporangiophore. (B) Rttizopus stoloniJer, habit 

study showing stages of development. (C) Thamnidiumetegans, primary 

sporangium and sporangioles. (D) Curmingtnmella sp., sporangiophore with 

monosporous sporangioles. (E) Choanephora cucurbitarum, 

conidiosporangia. (F) SUnceptwla-strum racetrLosum, sporangia and spores. 

(From Parbery, 1980.) 

Rhizopus soft rot of fruit and vegetables 

Rhizopus spp. are usually saprophytes but sometimes become weak parasites on 

fleshy tissues such as fruit, vegetables and propagating organs such as corrns 

and bulbs. They are important postharvest pathogens of crops such as cucurbits, 

sweet potatoes, stone fruits and many other fruits and vegetables. 

Rhizoptts usually gains entry into its host through wounds. Sporangiospores 

germinate when they land on wounds and the hyphae that develop secrete 

enz,yrnes which dissolve the middle lamella and cell walls of the host cells causing 

disintegration of cells and the formation of soft rot symptoms. 

As Rhizopt-ts is a wound pathogen, the incidence of disease can be reduced by 

minimising the amount of wounding of fruits and vegetables during harvest, 

handling, storage and transportation. Containers used for storing harvested 

products should be disinfected to remove viable spores and hyphae that might 

otherwise result in infection. Control of temperature and humidity in storage 

rooms can also be used to reduce disease incidence and severity. Wrapping 

susceptible fruit in paper impregnated with fungicide has also been used to 

control rhizopus soft rot. 

Endogonales and Glomales 

The fungi that form arbuscular mycorrhizas were originally placed in the order 

Endogonales, family Endogonaceae, which contained the single genus Endogone. 

However, in L974 the genus was revised and split into seven genera, some of 

which formed arbuscular mycorrhizas and ectomycorrhizas while others were soil 

saprophytes.

4. F\trEiusith aseptate hgphae andno dikaryophase 

59 

In 199O, Morton and Benny revised the taxonomy of arbuscular mycorrhizal 

fungi moving them from the Endogonales to the newly established order 

Glomales. The order Endogonales now contains the single genus Endogone, some 

species of which form ectomycorrhizas while others are saprophytes in soil. The 

genus Endogone commonly forms zygospores but little is known about its asexual 

states. It is not known to form arbuscular mycorrhizas. 

The order Glomales was established to incorporate fungi that form arbuscular 

mycorrhizas in association with plant roots. However, the position of these fungi 

in the Zygornycota is only tentative as no species in the order is known to form 

either zygospores or sporangiospores. Arbuscular mycorrhizal fungi are obli.gate 

parasites that form mutualistic symbioses with 8O-9Oo/o of all higher plants (see 

Chapter 2 for more detail on myconrbizal symbioses). They form large, thickwalled 

spores that are common in most soils. The genus Glomus produces spores 

that are 2O-4O pm in diameter. About 16O species belonging to six genera of 

arbuscular mycorrhizal fungi are recognised. 

4.5 Further reading 

Agrios, G.N. (19971. PlantPathologg. Academic Press, San Diego. 

Alexopoulos, C.J. and Mims, C.W. (19791. Introductory mgcotogg. John Wiley and Sons, 

NewYork and Brisbane. 

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Fungi with aseptate hyphae and no dikaryophase

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