P ests in agricultural plants 1985

C ereals a n d grasses

C ereal cyst nem atodes (H eterodera avenae). In June, 18% of th e advisers re p o rte d o f m edium severe to severe attacks, whereas, in July, th e attacks were described as weak to m edium severe. A t the R esearch C entre for P lant Protection an exam ination of about 250 spring barley fields was carried o u t which showed th a t th ere w ere an unacceptable large num ber of nem atodes in approximately 40% o f the fields.

B ird-cherry ap h id s (R h opalosiph u m p a d i) and g rain a p h id s (Sitobion avenae). T h e first aphids w ere seen in spring barley late in May, especially in the so u th ern p art of the country. In June, the attacks in spring barley w ere described as m oderate, b u t generally w idespread. O ut o f 72 reports, however, 12 cases w ere described as m edium severe to severe attacks. In July, the aphid attacks w ere generally described as no o r weak attacks. However, th e re w ere a few severe attacks in w inter w heat and spring barley, especially in the southern parts of the country. In experim ents carried out with a large n u m b er of aphids yield increases of up to 16 h k g /h a w ere obtained. Especially at high N-levels the aphids w ere persevering. In 1985, the parazitation of the aphids occurred rather late.

T he grain aphid, as so often before, was the dom inant species.

G rain th rip s (L im othrips cerealium and L . den tico m is). Several cases of g rain thrips attacks w ere found in 1985. T he characteristic w hite sheaths were seen especially in rye. In June, 13% of the advisers rep o rted o f m edium severe to severe attacks, w hereas the general rep o rts w ere o f no or weak attacks. In an experim ent with rye infested by m any thrips a yield increase o f 4.0 h k g /h a was o b tain ed after control.

W irew orm s (A griotes spp.). B oth the extension and the severity of the attacks w ere lim ited. 85% rep o rt of no attack.

L eatherjackets (Tipula p a lu d o sa ). T he attacks w ere w idespread in Jutland - especially in S outhern Jutland, N o rth ern Schleswig and C entral Jutland. O n Sealand th e re w ere lim ited attacks. L eatherjackets often occur in connection with spring b arle y and b eet following ploughed grass.

Italian rye grass in pu re culture was also severely attacked, and it was necessary to resow in several cases.

T h e reports described th e severity of the attacks as "no" (18% ), "weak" (16% ),

"medium strong" (37% ), and "severe" (29% ). A ccording to the w eather conditions in the autum n an d soil sam ples taken, weak attacks are expected in 1986.

F rit flies (O scin ella frit). T he first generation m igrated in the w eek 20th to 27th May, which w as later th an usual. This was caused by the very cold spring. D ue to the hard w inter th e population was expected to drop.

T he m igration of the first generation differed significantly from one locality to the other, b u t w as generally weak. O n the 10th of July w arnings against the second generation w ere sent out. This generation, however, took a long tim e because excessive rainfall ham pered the m igration, and the rath er low tem p eratu res delayed the hatching.

T he 3rd as well as th e 2nd generation w ere sm aller than usual.

T he attacks o f b o th the 1st, 2nd and 3rd generation w ere weak in 1985, and they were sm aller in num ber than in 1984.

Saddle gall m idges (H aplodiplosis equestris). T h ere w ere rep o rts of w eak attacks only. So far th e re is nothing indicating a fresh outbreak o f this pest.

Leaf beetles ( O u lem a m elan opu s) occurred with weak to m edium severe attacks.

(Ochsenheimeria vaculella) was found in spring barley in N orth-w estern Sealand.

Experiences w ith this pest which causes blasted spikelets are still limited.

W heat gall m idges ( Contarinia tritici and Sitodiplosis m osellan a). A ttacks in w heat were insignificant.

Bibionid flies (B ib io hortulanus). Extension and attacks w ere very limited.

Potato stem b o re rs (H ydraecia m icacea). O ne attack in m aize was rep o rted . T he attack was confined to the edge o f the field.

Slugs (A griolim ax spp.). In Septem ber - O ctober th ere w ere m any rep o rts o f severe attacks in w inter cereal crops and w inter rape. Resow ing was necessary in several cases. E specially w here w inter cereal crops and w inter rap e followed rape, peas, clover and grass th e re w ere reports o f attacks. Heavy, lumpy types of soil sown by means o f the ro to r harrow system had severe attacks. T he large slug population was ascribed to the favourable conditions for reproduction u nder ra th e r w et and cold w eather conditions during late sum m er and autum n.

Legumes

Cabbage th rip s (Thrips angusticeps). A ttacks w ere w idespread in C entral and S outhern Sealand and in N o rth ern Jutland. T he severity o f th e attack was generally w eak (78% o f the reports). O nly in one locality severe attacks w ere found.

Pea th rip s (K akothrips p isivo ru s). In July the attacks w ere described as n o n e o r weak.

Pea a n d bean weevils (S iton a lineatus). The occurrence was widespread, especially in C entral and S outhern Sealand, on Funen and in C en tral Jutland. In these a re a s the attacks w ere described as weak (61% ), medium strong (29% ) and severe (5% ).

Severe attacks by adults w ere seen on leaves, w hereas the dam age to root nodules by larvae could not be detected immediately. Severe larval attacks may resu lt in yellow wilted leaves, which may look like N-deficiency, because the larvae e a t th e N -containing ro o t nodules.

In A ugust th ere w ere rep o rts of weak to m edium severe attacks by pea and b e a n weevils in clover. 3 advisers rep o rted of severe attacks. In S eptem ber 61% o f th e advisers rep o rte d of m edium severe attacks. Severe attacks w ere seen in W estern Ju tlan d (3 rep o rts), Funen (2), L olland-F alster/S outheast Sealand (2), C entral and W estern Sealand (1).

C ontrol of pea and b e an weevils in weak white-clover was necessary in several cases. W ith low tem p eratu res in O ctober the pea and bean weevils hibernated.

1985 was the 2nd year in a row with severe attacks of p e a and bean weevils in the late sum m er in clover, probably due to a considerable increase in the areas grow n w ith peas, in which th e p ea and bean weevils can b e reproduced and attack the clover after the peas have been harvested.

T o rtrix m oth c a te rp illa rs (C nephasia spp.) w ere generally w idespread in peas in som e areas, but th ere w ere only 3 reports o f m edium severe attacks. In b ro a d beans th e re w ere only few and w eak attacks by tortrix m oth caterpillars.

Pea ap h id s (A cyrthosiphon p isu m ). In June pea aphids had only spread very little, and attacks w ere weak. In July 27% reported of m edium severe attacks, and p ea aphids w ere generally w idespread. In A ugust the attack and the spread h ad decreased, b u t th e re w ere few ra th e r late, severe attacks.

B lack bean a p h id s (A ph is fa b a e ) in field beans w ere generally widespread and o ccurred in July w ith m edium severe attacks i 9 out of 21 reports.

Pea moths (C ydia nigricana). M igration was so low this year, that control o f pea m oths was not necessary. 4 plant protection bulletins on the pea m oth w ere sent out. During th e first week of Ju n e th ere w ere p ea m oths in 2 out of 180 traps distributed all over the country. Beginning and M id July p ea m oths w ere on the wings in approxim ately 15% of the localities. It was not possible to detect any difference in m igration from one region to another by looking at the figures.

O nly the m igration activity was higher on Sealand in Ju n e and July (50% o f the localities with initial m igration), and partly in Ju tlan d (W est, E ast, N orthern Jutland).

R eports from th e advisers also showed weak attacks in June, July and August.

Flower weevils (A p io n spp.) in clover occurred in few fields and generally with w eak attacks. O nly from N orthern Jutland th ere w ere reports o f severe attacks.

Oilseed rape

Blossom beetles (M eligethes aeneus) were w idespread in w inter rap e fields in May. The cold w eath er at the beginning of M ay ham pered the m igration of blossom beetles so m uch th a t attacks in w inter rape w ere m oderate. L ater in the m onth the tem perature rose, and in June w hen the spring rap e is flowering the attacks were described by 70% o f the advisers as w idespread and with m edium severe to severe attacks.

Seed weevils (Ceutorrhynchus assim ilis). By the end o f M ay the m ain part o f the winter rape fields had been infested by the seed weevils with varying severity.

A lm ost half o f th e advisers estim ated the attacks as m edium severe.

T he ocurrence of seed weevils, as was the case for brassica pod midges, was also very w idespread in Ju n e and m ore than half of the advisers estim ated the attacks as medium severe. In July, apart from few severe attacks, only weak to m edium severe attacks w ere seen in w inter rape and no or weak attacks in spring rape.

Brassica p o d m idges (Dasyneura brassicae). T he brassica pod m idge m igrated by the end of M ay. O n the 28th of M ay warning against the 1st generation of the brassica pod m idge was sent out and control was necessary approxim ately 5 days later.

The overw intering conditions for the brassica pod m idge w ere good in 1984/85.

Therefore th e re was a risk of m oderate to severe attacks in areas which the year before h ad had a large population o f brassica pod midges (N orth Schleswig, Northern F u n e n and S outhern Sealand). In Ju n e th ere w ere rep o rts of very widespread attacks by brassica pod midges in w inter rape with m edium severe

attacks in m ore than half o f the reports. In July the attacks were generally w eak to m edium with few severe attacks, however. In spring ra p e there were generally no or weak attacks.

T h ere w ere rep o rts of insufficient control o f brassica p o d midges in sp ite of several sprayings w ith pyrethroids, probably because th e control was ap p lied too late or due to a vigorous vegetation (com bined with a large density o f brassica pod m idges) resulting in quicker dilution of pyrethroids in the rape p lan ts than usual.

W arning against the second generation of brassica pod m idges was sent o u t on the 27th of June. In m ost places evidence of the occurrence o f the first g en eratio n of brassica pod m idges was found in w inter rape. In fields w ith insufficient co n tro l the attacks w ere severe (30 - 40% of the pods w ere destroyed.

Flea beetles (P hyllotreta spp.). Spread and attacks w ere very lim ited.

C abbage stem flea beetles (Psylliodes chrysocephala) did n o t cause problem s, one adviser in S outhern Sealand, however, decribed the attack as m edium strong.

Cabbage th rip s (Thrips angusticeps) did not cause problem s in winter ra p e . In spring rap e the occurrence was m oderate all over th e country. In a few places in S outhern Sealand th e re w ere cabbage thrips in m ost o f th e fields. In M ay, the attacks w ere estim ated as w eak (68% ) or m edium severe (21% ), whereas o n e ad­

viser reports on severe attacks. T he spread and the attacks seem ed to b e m o re severe than in 1984.

C abbage stem weevils (Ceutorrhynchus quadridens). T h ere w ere generally n o o r weak attacks. H ow ever, th ere w ere several cases o f severe attacks. The cabbage stem weevil attracted m ore attention in 1985 than in previous years. T h e light, wilted stem s that the cabbage stem weevil leaves behind, w ere in a few cases m istaken for sym ptom s o f sclerotinia disease.

C abbage ro o t flies and tu rn ip ro o t flies (D elia radicum , D . floralis) o ccurred in w inter and spring rap e with only few attacks of no or weak strength.

D iam ond-back m oths (Plutella xylostella) occurred very sporadically and w as of no im portance.

Swede gall m idges (C ontarinia nasturtii). 34 advisers observed no attacks, w hile 10 advisers saw weak attacks and only one reported on m edium severe attacks.

C abbage bu tterflies (Pieris brassicae, P. rapae). T h e re w ere few w eak attacks.

Cabbage a p h id s (Brevicoryne brassicae) was less w idespread and with weak attacks.

Root crops.

a~> Beet

Beet nem atodes (H eterodera schachtii). Only 5 rep o rted o f occurrences - there were both w eak and severe attacks.

Cabbage th rip s (Thrips angusticeps). A s in 1984 th ere w ere w idespread attacks in beet in 1985. H ow ever, the attacks w ere much weaker, and resow ing was unnecessary.

In May, approxim ately 40 advisers estim ated the extent and dam aging effects of the following pests (percentage):

PESTS ATTA CK S

no weak m edium severe

Wireworms 22 64 11 3

Pygmy beetles 58 39 3 0

Collembola 71 21 8 0

Millipedes 76 24 0 0

Beet leaf m iners 71 11 14 4

Pygmy beetles (A tom aria linearis) occurred as in 1984 in a few fields, w here the individual cases gave rise to dam age.

Beet leaf m in ers (P egom yia hyoscyam i). Spread and severity was in M ay as in previous years, an d it was found in approxim ately 20% of the fields, o f which a few had severe attacks. In A ugust 4 advisers had seen severe attacks.

Beet carrio n beetles (B litophaga opaca) was m ore w idespread than in 1984, but only 8 advisers out of 69 rep o rted of medium strong attacks.

T ortrix m oth larv ae (Cnephasia spp.) w ere w idespread, b u t only 3 advisers saw medium strong attacks.

N ut meg m oth (D icestra trifolii) and cabbage m oth (M am estra brassicae) occurred in several fields, b u t with weak attacks. Even though the nut m eg m oths prefer dry conditions they occurred with severe attacks in N orthern Jutland. 1985 is the third year in a row with severe attacks by larvae o f the nut m eg m oth in beet in Northern Ju tlan d .

C utw orm s (Agrotis segetu m ). D u e to the wet w eather conditions, attacks w ere less w idespread and generally weak.

Peach p o ta to a p h id s (M yzus persica e) and black bean a p h id s {Aphis f a b a e ) . In 1985, 7 plant protection bulletins concerning aphids in b e e t were sent o u t in Ju n e - July.

T he bulletins w ere based on rep o rts from the advisers on th e num ber of p each p otato aphids and black b ean aphids registered in beet fields all over the country.

A t th e beginning of July a w arning was sent out for control of peach p o ta to aphids on M øn, S outhern Sealand and Lolland-Falster. O n e week later co n tro l was necessary on Funen. F rom several areas too little inform ation had b een sen t in during this period as to the num ber of peach p o tato aphids in the fields exam ined, and therefore it was not possible to estim ate th e need for control, if any. Especially from areas with m any late and infested b eet pits reports w ere few.

O ut o f 40 spindle tree localities exam ined in the spring, black b ean aphids o ccu rred in 3 localities. In 1984, no overw intered eggs were found on spindle trees.

A t the beginning o f June, black bean aphids w ere w idespread in the so u th ern parts of the country and from the end of June until the beginning of July th e re w ere w idespread, but weak attacks in most parts. By the end o f June and as late as by the end of July th ere w ere severe attacks on the islands and in the so u th ern parts of the country, at th at tim e th ere were no signs o f parasiting yet. C o n tro l caused som e problem s in the southern parts. A lso in th e experim ents th e b a d effect o f several o f the system ic phosphorous agents was observed, perhaps du e to resistance developing.

T h e black b ean aphids occurred late in 1985, b u t with ra th e r severe, late attacks on the islands and in the sou th ern parts o f the country.

b) Potatoes

P o tato cyst nem atodes (H eterodera rostochiensis). Two advisers rep o rte d of attacks.

C utw orm s (Agrotis segetu m ). T he turnip m oths m igrated later this year than usual- not until 10th July did the phero m o n e traps catch many anim als.

C onsidering the rainfall and tem perature conditions, severe attacks by cutw orm s w ere not expected. A plant protection bulletin was sent o u t o n the 25th o f Ju n e . O n the 16th o f July the second plant protection bulletin was sent out. T he catch of turnip m oths decreased in Sealand and Funen. O n Sam sø a decrease late in

June was follow ed by a considerable increase in catches. In Jutland catches w ere generally large except in Southern Jutland.

C olorado beetles (L eptinotarsa decem lin eata) w ere observed in 1985 with severe attacks in a new locality, a private garden in Lolland-Falster. They had probably overwintered from 1984. A lso from T ø n d er th ere w ere rep o rts o f the occurrence o f 19 sum m er beetles in a private garden. In 1985 th ere w ere a total o f 4 rep o rts on the occurrence o f Colorado beetles.

3. D iseases a n d p ests o f h o rtic u ltu ra l p la n ts 1985 (L ars A. H obolth)

V ariations are few in the percentual distribution o f enquiries when com pared to an average o f the five preceding years (table 1). T he enquiries tend, how ever, to b e directed m o re tow ards specific subjects, and so the n u m b er of enquiries on mycology, bacteriology, virology and zoology has increased a little.

T able 1.

P ercentual distribution of registered enquiries

Phy- Myco- B acte­ Viro- Z o o ­ Unex­

sioge- logi riolo­ logi- logi­ D i­ plain­

nical cal gical cal cal verse ed

A verage 14.8 41.8 7.5 4.9 24.5 5.8 0.6

of 5 years 1980-84

1985 10.1 44.2 9.2 6.0 27.2 3.7 0.06

(total num ber: 1672) C lim atic dam age

W hen spring was well advanced, m any trees and bushes show ed signs o f injuries caused by the w inter. M any roses, b o th the old plants and th e grafted ones died.

A pparently th e grafted plants th at h ad been ridged up a fter the grafting h ad taken place suffered fewest injuries.

M any tulips w ere dam aged by too m uch ethylen, possibly d ue to the cold sto res having been too carefully locked to keep out the cold.

T he late spring h am p ered the developm ent of Chinese cabbage when it was p lan ted out, so that m any plants form ed flowers before heads. B ecause o f the cold w e ath e r in Ju n e m any outd o o r cultures requiring warm w eather sto p p ed developing. T his w as th e case f.inst. of sw eet corn, large cucum ber and gherkins. In straw berries th e w eather resulted in an insufficient pollination o f the b erries, and this explains th e large num ber o f deform ed straw berries found. L ate r in th e summer several local hailstorm s caused dam age to various crops. In onion th e skin was dam aged so that different fungi gained access to the onion.

Fungal diseases

Clubroot (P la sm o d io p h o ra brassicae) is still found now and then in plantlets. T he symptoms o ften ap p ea r so late that the plants have already been planted in the field, before th e sym ptom s can be seen. C onsidering the vigorous developm ent o f the fungus in th e course o f one to two weeks after the planting had taken place there can b e no doubt that they w ere infected already during the germ ination.

A s seed transm ission o f this disease is not known of, suspicion m ust be directed towards the cultivation m edium . Infected fields have been found, w here the previous crop was not cabbage but o ther cruciferae crops, for instance oilseed rape and m ustard. T hese crops may cause problem s, because attacks m ust be very severe to b e detected, as the roots a re not lifted during the harvest, which would reveal the typical swellings of the roots.

Pythium spp. destroys the roots of m any glasshouse cultures. T h e slightest imperfection o f th e cultivation is often the beginning o f an attack o f the fungi.

This could b e a to o com pact standard soil, causing the roots to suffocate, o r a tem porary im perfection o f the irrigation system, creating a conductivity so high that the roots a re destroyed at the slightest drying up o f the soil.

Phytophthora spp. is just as w idespread as Pythium in pot plant nurseries. This fungus is often detected m ore quickly, because frequently it is developing up into the plants, so th a t the infection can b e seen over the soil surface, and the injuries caused by it a re m o re drastic than those caused by Pythium .

Leathery ro t (P ytophth ora cactorum ) was found several tim es in straw berry cultures. T he typical sym ptom is a num ber o f brownish, coriaceous berries in the clusters.

White Tip of leek (P hytophthora porri) is a com m on disease in m any leek cultures, perhaps because this culture, like m any o th er cultures, is grow n in an insufficient crop rotation.

Downy mildew of onion (P eron ospora destructor) was found som ew hat later in 1985 than w as th e case the preceding year. T he extension of the attack seem s to

Downy mildew of onion (P eron ospora destructor) was found som ew hat later in 1985 than w as th e case the preceding year. T he extension of the attack seem s to

In document Plant diseases, pests and weeds in Denmark 1985 (Sider 24-0)